1. Field of the Invention
The present invention relates to an automatic focusing device and, more particularly, to an automatic continuous focusing device wherein two detected images of an object formed by a distance detecting optical system with a relative distance therebetween associated with the object distance are scanned by a scanning means, the scanning signals thus obtained are quantized by a quantizing means to generate quantized picture element data, the relative distance between the two detected images is detected by operation means on the basis of the quantized picture element data, and an imaging optical system to be focused on the object is controlled by a control means for focusing it according to the detection result.
2. Description of the Prior Art
Some automatic focusing devices of the type described above have been proposed. For example, in the specifications of U.S. Pat. Nos. 4,004,852 and 4,078,171 (corresponding Laid-open Japanese patent application No. 51-153433), examples of an automatic focusing device are disclosed according to which two detected images formed by a distance detection optical system of stationary base line distance measuring unit type having no moving parts with a relative distance therebetween corresponding to the object distance are scanned by a solid image pickup element such as a line type image sensor, the scanning picture element signals thus obtained are converted into binary signals (quantized) by a binary encoder (quantizing means) to generate binary picture element data (quantized picture element data) consisting of signals of logic values "0" and "1", the relative distance between the two detected images is detected by a digital arithmetic means according to the binary picture element data to obtain an object distance, the data on the object distance is compared with data on the position of an imaging optical system to be focused on the object on the optical axis to generate control signals, and the imaging optical system is controlled by a control means including drive means such as a motor for focusing it on the object in response to the control signals. In particular, in the device disclosed in the U.S. Pat. No. 4,004,852, a method is adopted according to which data on an image of a first field of view as a reference with respect to an object for detection of an object distance and data on an image of a second field of view wider than the first field of view (number of data handled for the first and second fields of view are different and the number of data for the second field of view is greater than the other) are obtained, and a position of an image part in the second field of view corresponding to the image in the first field of view or most resembling it is detected to thereby obtain distance data with respect to the infinite object distance as a reference.
Unlike the device disclosed in U.S. Pat. No. 4,004,852 described above which adopts the method of comparing data on the position of the imaging optical system with the obtained absolute distance data, U.S. patent application Ser. No. 944,974 filed on Sept. 22, 1978, now U.S. Pat. No. 4,305,657 (corresponding to British Patent Application Publication No. 2009553) of the present assignee discloses an example of an automatic focusing device according to which part of the system associated with the second field of view is movable and is finely moved on a solid pickup element in association with the control of the imaging optical system so that the image part of the second field of view corresponding to the image of the first field of view or resembling it may be situated at a predetermined position, whereby focusing of the imaging optical system may be accomplished.
In a device of this type, particularly since the scanning signals of the detected images of an object are quantized to generate the quantized picture element data and the distance detection or the focus detection is performed according to this quantized picture element data, correct detection may not be achieved when the contrast of the object is low, resulting in very unstable control of the imaging optical system and occasionally in very erratic operation. When the contrast of an object is low, the possibility increases that the binary data which is obtained by slicing at a predetermined threshold level may all be converted to the same data, that is, the possibility of conversion to signals of a single logic value "1" or "0" increases. When such a phenomenon occurs, correct detection becomes impossible. Especially when a solid pickup element is used as has been described hereinabove, non-uniformity of the sensitivity of the photosensor elements also becomes a big factor, contributing to the phenomenon described above.
Especially with a camera for continuous photography such as an 8 mm movie camera or a video camera, the subject changes continuously. Therefore, when an automatic focusing device as described above is mounted to a camera of this type, and when the subject changes from an object of high contrast to an object of low contrast or of solid color, correct detection may be performed with the object of high contrast but correct detection may or may not be performed with the object of low contrast or of solid color, resulting in very unstable control operation of the photographic lens and occasionally very erratic operation.